The present invention relates to a device for providing a focus signal at the point of focus of a photographing lens on an object to be photographed in a device which automatically detects focus by detecting the contrast of the image.
When the image of an object to be photographed is precisely focused by the photographing lens of a camera, the contrast of light and shade of the image becomes a maximum. This is due to the fact that the power spectrum (optical intensity) of the spatial frequency spectrum of the image of the object becomes a maximum at the point of focus. A number of automatic focus detecting devices have been proposed which utilize this phenomenon.
Such devices are generally structurally simple. More specifically, in such a device, a contrast detecting photoelectric element is disposed in a plane which is at a position equivalent to the plane of the surface of a film and a single contrast signal is produced in response to an object image projected onto the photoelectric element. The contrast signal thus detected becomes a maximum at the position of focus. That is, the characteristic curve of the amplitudes of the contrast signals plotted against the extension of the photographing lens has a single peak. Accordingly, in order for the device to accurately detect the focus position, it is necessary to provide means for detecting the maximum value of the contrast signal.
One example of such a detecting means is a measuring instrument with a pointer which is so designed that the deflection of the pointer is in porportion to the magnitude of the input contrast signal. With this measuring instrument, the photographer can visually detect the point maximum contrast from the maximum deflection of the pointer. However, it is rather troublesome for the photographer to detect the maximum deflection of a pointer. If it is possible to produce a focus signal in such a manner that a lamp is turned on with the aid of the focus signal only at the position of focus then the photographer can readily determine that the image of an object to be photographed is focused. However, it should be noted that it is difficult to produce a focus signal from a single contrast signal as described above because different contrast signals are obtained from different objects and accordingly the contrast signal produced at the position of focus is not always constant.
Production of a focus signal from a single contrast signal as described above may be achieved by a technique in which the maximum value of a contrast signal is stored and, when the contrast signal reaches the same maximum value again, the former and latter contrast signals are compared and, when they are equal, the focus signal is generated. However, this technique suffers from two serious problems. One of the two problems is that, if the photographing lens is reversed in direction before it reaches the true focalization position, the contrast signal apparently shows a maximum value. That is, a false peak signal is provided whereby a false focus signal is produced. The second problem is that, before the image of a new object is focused, it is necessary to reset or clear the value which has been stored for the previous object.
Because of the above-described difficulties, effective focus signals cannot be provided by conventional devices which simply extract a contrast signal from the image of an object. In order to overcome these difficulties, several improved techniques have been proposed in the art. In one of devices for practicing the conventional methods, two contrast signals produced by two contrast detecting photoelectric elements which are spaced from each other in the direction of optical axis are compared to obtain a focus signal. In another device, a contrast detecting photoelectric element is vibrated in the direction of the camera's optical axis to produce contrast signals from at least two object images and the contrast signals are compared to obtain a focus signal similar to the above-described device. However, such a device in which the contrast signals of two object images spaced in the direction of optical axis are extracted spatially or by time division is generally large in size and high in manufacturing cost.
Accordingly, an object of this invention is to eliminate the above-described difficulties. More specifically, an object of the invention is to provide an automatic focus indicating device in which a single contrast signal is utilized and in which a light emitting element is turned on at the position of focus. The device of the invention is meritorious in that it is compact and low in manufacturing cost and the point of focus can be readily determined.